1. Field of the Invention
The disclosed technology relates to a method for forming an organic material layer on a substrate in an in-line deposition system, to an organic material layer such obtained, to the use of such a method in a process for forming an organic thin film transistor, to an injector for use in such an in-line deposition system, and to an in-line deposition system for use in such a method.
2. Description of the Related Technology
The industrial production of devices and circuits based on small organic semiconducting molecules requires the availability of high throughput deposition techniques to grow such films. High throughputs require a technology that allows depositing a large amount of organic material over large area substrates while ensuring a good optical and/or electrical quality and a good uniformity of the deposited films. To reach such a goal, a commonly proposed approach is reel-to-reel (or roll-to-roll) processing, in which small organic semiconducting molecules are distributed by a linear elongated source over a continuously moving substrate.
For the growth of thin films based on small organic molecules in reel-to-reel conditions, two techniques have been proposed: in-line organic molecular beam deposition (OMBD, also referred to as vacuum thermal evaporation) and in-line organic vapor phase deposition (OVPD).
In-line OMBD is a high vacuum process in which the organic molecules are thermally evaporated from an elongated source. The evaporated molecules travel in the molecular flow regime towards a temperature-controlled substrate on which they condense to form a thin film. The substrate and the elongated source are in relative motion in a direction perpendicular to the direction of elongation of the source. The elongated sources are typically sealed with lid structures having a plurality of apertures, the size, shape and spacing of which can be adjusted to meet uniformity requirements, as e.g. described in US 2007/0163497.
An OVPD process uses an inert carrier gas to transport organic molecules from a source cell onto a cooled substrate in a hot-walled low-pressure chamber. The carrier gas convectively transports the organic molecules away from the source towards a temperature controlled substrate on which the organic molecules condense to form a thin film. OVPD can be preformed in an in-line system in which the loaded carrier gas is distributed by an elongated injector onto the substrate. The substrate and elongated injector are in relative motion in a direction perpendicular to the direction of elongation of the injector. Several geometries for the elongated injector have been proposed. The most convenient one is the showerhead, consisting of a plate with a plurality of openings or apertures through which the carrier gas flows towards the substrate. In-line OVPD deposition systems are for example referred to in U.S. Pat. No. 6,337,102 and in US 2005/0109281.
The extension of a static organic layer deposition system to an in-line system with a reel-to-reel geometry influences the deposition rate profile of the organic layers. The deposition rate profile can be defined as the deposition rate as a function of time during the deposition process at a given location on a substrate. In static processing systems the deposition rate can easily be held constant during the whole deposition process by ensuring a constant rate of evaporation of material at the source. This method gives rise to a square-shaped deposition rate profile, with abrupt transitions from no deposition to deposition and from deposition to no deposition, and with a constant deposition rate during deposition. However, in an in-line geometry the relative motion between the substrate and the elongated organic molecules injector (e.g. showerhead) is a source of variation of the deposition rate. At a location on the substrate that is far away from the injector, the deposition rate is zero. At a location on the substrate in front of the injector, the deposition rate is at its maximum. In between these points the deposition rate varies according to a deposition rate profile. In order to mimic the deposition rate profile obtained in static systems, the different parts of an in-line deposition system are in general designed such that the deposition rate profile has a shape that is as square as possible.
A high throughput reel-to-reel processing tool may be able to continuously coat a substrate moving at a constant speed (further referred to as ‘substrate speed’) of e.g. 1 m/min or more. In case of deposition of a layer by means of an in-line system, e.g. a reel-to-reel system, the linear deposition speed can be used as a defining parameter. The linear deposition speed can be defined as the product of the deposited thickness and the substrate speed. It can be expressed in micrometer2/s. For example, when an in-line production tool with a substrate speed of 1 m/min is used for depositing a 30 nm thick organic layer on the substrate, a linear deposition speed of 30 nm×1 m/min=500 micrometer2/s is needed. It can be shown that the linear deposition speed at a given point on the substrate equals the product of the substrate speed with the integral of the deposition rate profile over the whole period of the deposition.
It is an advantage of a reel-to-reel geometry that it allows high throughput production of uniform organic films, for example for fabricating OLEDs (Organic Light Emitting Devices). A reel-to-reel system is considered to be a high throughput system if it is able to continuously coat a substrate moving at a substrate speed above 1 m/min. However, high deposition rates lead to organic films of a poor electrical quality. When such films are used for fabricating OTFTs (Organic Thin Film Transistors), this leads to devices with low charge carrier mobility and thus poor quality. For example, in “Pentacene-based organic field-effect transistors”, M. Kitamura et al., Journal of Physics: Condensed Matter 20 (2008) 184011, report that the grain size of pentacene thin films deposited by thermal evaporation decreases with increasing deposition rate. The grain size in pentacene films significantly influences carrier transport in these films. The mobility increases monotonically with the grain size. In “Influence of grain sizes on the mobility of organic thin film transistors”, Applied Physics Letters 86, 263501, 2005, A. Di Carlo et al. report that, for a given substrate temperature, the size of the grains is larger for lower deposition rates. High deposition rates deliver very small grains. Moreover, a strong dependency of the mobility on the grain size is reported. It is shown that the field-effect-extracted mobility abruptly drops for a grain size smaller than 2 micrometer. Therefore, the best transistor characteristics are usually obtained with films deposited at a rather low deposition rate (and thus a large grain size).
With a static OMBD system, the best Organic Thin Film Transistors (OTFTs) are made with films grown at deposition rates below 0.25 Å/s. This leads to a total deposition time of 1200 s for a 30 nm thick film. Such long deposition times are unpractical if one aims at the fabrication at an industrial scale of organic circuits comprising OTFTs. Using a static OVPD system, good pentacene OTFTs are reported with deposition rates up to 9.5 Å/s (C. Rolin et al, “Pentacene devices and logic gates fabricated by organic vapor phase deposition”, Applied Physics Letters 89, 203502 (2006)).
Definitions
In-line deposition system: a system for depositing a layer of a material on a substrate, wherein the material is provided through a linear elongated injector and wherein the substrate and the linear elongated injector are in relative movement.
Substrate speed: speed of a substrate relative to an injector.
Longitudinal direction of a linear elongated injector: the direction substantially orthogonal to the direction of substrate movement.
Length of a linear elongated injector: size of the injector in the longitudinal direction of the injector.
Width of a linear elongated injector: size of the injector in a direction substantially orthogonal to the longitudinal direction and in the plane of the injector.
Thickness of a linear elongated injector: size of the injector in a direction substantially orthogonal to the plane of the injector.
Front edge and back edge of a linear elongated injector: sides of the injector substantially orthogonal to the direction of substrate movement, wherein the front edge is the edge where a given point of the moving substrate enters the deposition zone underneath the injector, and wherein the back edge is the edge where a given point of the moving substrate leaves the deposition zone underneath the injector.
Distance between the front edge and the back edge of the injector: the width of the injector.
Symmetric injector: an injector that comprises two parts that are identical mirror images of each other, wherein the axis of symmetry is oriented along the longitudinal direction of the injector.
Asymmetric injector: injector that cannot be divided along its longitudinal direction into two halves that are identical minor images of each other.
Deposition rate profile: the deposition rate as a function of time during a deposition process at a given location on a substrate. A typical deposition rate profile shows a rising edge, followed by a period with a maximum deposition rate and a falling edge or trailing edge. The rising edge is characterized by an increase of the deposition rate from zero to the maximum value. The falling edge or trailing edge of the deposition rate profile is characterized by a decrease of the deposition rate from the maximum value to zero. In between the rising edge and the trailing edge the deposition rate can be constant or it can vary. A symmetric deposition rate profile is a profile that comprises two parts that are identical mirror images. For example, in a symmetric deposition rate profile the trailing edge is the mirror image of the rising edge. An asymmetric deposition rate profile is a deposition rate profile that can not be divided into two halves that are identical minor images of each other.
Linear deposition speed of an in-line deposition system at a given point of a substrate: the product of the substrate speed and the (final) thickness of a layer or film deposited at that point of a substrate. The linear deposition speed at a given point on the substrate equals the product of the substrate speed with the integral of the deposition rate profile over the whole period of the deposition. It can be expressed in micrometer2/s.
Average deposition rate of an in-line deposition system at a given point on a substrate: the average of the deposition rate over a certain period of time at that point of the substrate.
Material utilization efficiency: the ratio between the amount of material (in moles) deposited on the substrate and the amount of material (in moles) evaporated from the source. The higher this efficiency, the less material is wasted during the process.
Showerhead: a plate with a plurality of openings or apertures through which a carrier gas flows towards the substrate.